Recently, higher color image production rates employing electrophotography have been demanded. In order to realize the above higher rates, sought is a toner capable of consistently producing color images during color image production at a higher rate.
However, when color images are produced at a higher rate, period passing the fixing nip section in a fixing device decreases resulting in a decrease in pressurizing/heating energy provided to the toner, whereby it has been difficult to consistently produce color images due to occasional formation of image defects due to insufficient image fixing, such as the offsetting phenomena.
On the other hand, energy conservation in an image forming apparatus employing an electrophotographic method has also been demanded. In the above image forming apparatus, to decrease energy consumption in the most power consuming fixing unit, research for a method to achieve fixing at a lower fixing temperature have been conducted. In order to realize lower temperature fixing, it is necessary to melt toner particles and releasing agents at lower temperatures. In order to archive the above, it is commonly required to employ components exhibiting lower melting viscosity as a toner and a releasing agent (being wax), and also to employ components exhibiting a lower melting point as the releasing agent. Thus, toners are proposed which employ such releasing agents (hereinafter also referred to as specified lower melting point releasing agents) exhibiting lower melting viscosity and low melting point (refer, for example, to Patent Documents 1 and 2).
However, fixed images, which are formed employing toners incorporating such specified lower melting point releasing agents, result in problems in which image defects in the form of banding and streaking tend to occur.
Causes of the above problems are studied to result in finding in which molecules of the releasing agents adhere to the interior of the apparatus to adversely affect charging properties, and to also result in mirror staining. Inherently, releasing agents themselves exhibit a low melting point but a very high boiling point, whereby it has not been assumed that they vaporize. However, it was assumed that in order to realize lower temperature fixing, as the melting point of releasing agents is lowered, the vapor pressure at lower than or equal to the boiling point decreases to result in an increase in vaporized molecules of the releasing agents or molecules having an easily vaporized structure. Namely, it was found that image defects occurred in the following manner. When images were formed, via thermal fixing, employing toners incorporating low melting point releasing agents, vaporized components were generated, from heat in the apparatus, due to the fact that the low melting point releasing agents themselves incorporated relatively easily vaporized components, whereby the above vaporized components adhered to the wires of the charging units to result in non-uniform charging, or onto polygonal mirrors to result in streaking defects during exposure.
(Patent Document 1) Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 2000-321815
(Patent Document 2) JP-A No. 2000-275908
In view of the foregoing, the present invention was achieved. An object of the present invention is to provide an image forming method capable of forming excellent fixed images in such a manner that by employing a toner incorporating low melting point releasing agents, even under high rate fixing, sufficiently strong fixing is carried out to minimize formation such as offsetting phenomenon and also to minimize image defects in the form of banding or streaking in the resulting fixed images.
In a fixing device employing a high-rate contact heating system, when a fixing temperature is elevated 50-100° C. higher than the melting point of the releasing agent so that the releasing agents function sufficiently, generation of vaporized components tend to be accelerated in such a manner that the releasing agents are subjected to effects due to heat. The inventors of the present invention found that a releasing agent, which exhibited a melting point nearly the same as the fixing temperature, resulted in difference in the amount of vaporized components. It was assumed that the cause was due to the intertwined structure of the releasing agent. Subsequently, the present invention was accomplished by specifying a dynamic viscosity of the releasing agent.
Though the reason is not well understood, it is assumed to be as follows. It is possible to regard the dynamic viscosity as an index of the relative ease of motion of molecular chains. By controlling the dynamic viscosity within a specified range, it is possible to limit the motion of molecular chains within that range. As a result, since it is possible to design in such a manner that when the components of releasing agent are thermally affected in the apparatus, a state is maintained in which molecular motion is not activated, and when thermally affected in the fixing device, the molecular motion is activated, it is possible to retard generation of vaporized components while maintaining the low temperature fixability.
The image forming method of the present invention is one in which a fixed image is prepared in such a manner that a toner image formed on the image support employing toners is fixed in the fixing nip section of the fixing device, employing a contact heating system. Toner, employed herein, contains a releasing agent in which:    the dynamic viscosity of the releasing agent is 4-20 mm2/second; and    the fixing temperature in the fixing nip section of the above fixing device is 50-100° C. higher than the melting point of the releasing agent, and particularly 75-100° C. higher than the melting point of the releasing agent.
In the image forming method of the present invention, the melting point of the above releasing agent is preferably 50-100° C. Further, the dynamic viscosity of the above releasing agents is preferably 8-15 mm2/second. Still further, it is preferable that the above releasing agent is composed of at least two types of releasing agents which exhibit a dynamic viscosity of 8-15 mm2/second.
In the image forming method of the present invention, the transport rate (hereinafter also referred to as the “printing rate”) in the fixing nip section is preferably 230-500 mm/second.
Based on the image forming method of the present invention, releasing agents contained in an employed toner exhibit a specified dynamic viscosity and at the same time, the melting point of the above releasing agent is within the specified range with respect to the fixing temperature in the fixing device. Consequently, even though fixing is carried out at a high rate, the releasing agents sufficiently melt to retard generation of phenomena such as offsetting, and minimize generation of vaporized components of the releasing agents to minimize formation of image defects in the form of banding and streaking on the fixed images, whereby it is possible to produce excellent fixed images.